Consider the following examples.
"... The Great Glass Elevator is shockproof, waterproof, bombproof, bulletproof, and Knidproof1..."
(From Charlie and the Great Glass Elevator by R. Dahl)
Lady Malvern tried to freeze him with a look, but you can't do that sort of thing to Jeeves. He is look-proof.
(From Carry on, Jeeves by P. G. Wodehouse)
Better sorts of lip-stick are frequently described in advertisements as kissproof. Some building materials may be advertised as fireproof. Certain technical devices are foolproof meaning that they are safe even in a fool's hands.
All these words, with -proof for the second component, stand between compounds and derived words in their characteristics. On the one hand, the second component seems to bear all the features of a stem and preserves certain semantic associations with the free form proof. On the other hand, the meaning of -proof in all the numerous words built on this pattern has become so generalized that it is certainly approaching that of a suffix. The high productivity of the pattern is proved, once more, by the possibility of coining nonce-words after this pattern: look-proof and Knidproof, the second produced from the non-existent stem Knid.
The component -proof, standing thus between a stem and an affix, is regarded by some scholars as a semi-affix.
Another example of semi-affix is -man in a vast group of English nouns denoting people: sportsman, gentleman, nobleman, salesman, seaman, fisherman, countryman, statesman, policeman, chairman, etc.
Semantically, the constituent -man in these words approaches the generalized meaning of such noun-forming suffixes as -er, -or, -ist (e. g. artist), -ite (e. g. hypocrite). It has moved so far in its meaning from the corresponding free form man, that such word-groups as woman policeman or Mrs. Chairman are quite usual. Nor does the statement Lady, you are no gentleman sound eccentric or illogical for the speaker uses the word gentleman in its general sense of a noble upright person, regardless of sex. It must be added though that this is only an occasional usage and that gentleman is normally applied to men.
Other examples of semi-affixes are -land (e. g. Ireland, Scotland, fatherland, wonderland), -like (e. g. ladylike, unladylike, businesslike, unbusinesslike, starlike, flowerlike, etc.), -worthy (e. g. seaworthy, trustworthy, praiseworthy).